In recent years, in order to cope with a high increase in the price of crude oil and in order to guard against the depletion of oil resources that may occur in near future, the following wells have been extensively developed worldwide: deep oil wells that have not attracted much attention and sour gas wells which are highly corrosive and of which the development has therefore been abandoned once. Steel pipes used for such oil wells and gas wells must have high corrosion resistance.
In environments containing a large amount of, for example, carbon dioxide, inhibitors have been used to prevent corrosion. However, the use of such inhibitors causes an increase in cost and the inhibitors cannot provide sufficient advantages under high temperature conditions in some cases. Therefore, steel pipes with high corrosion resistance have been recently used without using the inhibitors.
The API standards specify that 12%-Cr martensitic stainless steel with a reduced C content be suitable for line pipes. In recent years, martensitic stainless steel pipes have been used for pipelines for natural gas containing CO2. There is a problem in that such martensitic stainless steel pipes must be preheated or subjected to post-welding heat treatment when they are girth-welded. Furthermore, there is a problem in that welded portions thereof are inferior in toughness.
In order to cope with such problems, for example, Japanese Unexamined Patent Application Publication No. 9-316611 discloses martensitic stainless steel having a C content of 0.02% or less, an N content of 0.07% or less, an appropriate Cr content, an appropriate Ni content, and an appropriate Mo content. The Cr content, the Ni content, and the Mo content are adjusted in relation to the C content or the C content and the N content and the Ni content and the Mo content are adjusted in relation to the C content and the N content. A martensitic stainless steel pipe manufactured using the steel disclosed in this document is superior in CO2 corrosion resistance, resistance to stress corrosion cracking, weldability, and high-temperature strength and the toughness of a welded section of the pipe is high.